Method of preserving a rubber with an alpha, alpha&#39;-bis(hydroxyphenyl)-2, 6-xylenol ad resulting compositions



United States Patent METHOD OF PRESERVING A RUBBER WITH ANat,oz'-BIS(I'IYDROXYPHENYL)-2,6-XYLENOL AND RESULTING COMPOSITIONS DavidJ. Beaver, Richmond Heights, and Paul J. Stotfel,

Florissant, Mo., assignors to Monsanto Chemical Company, St. Louis, Mo.,a corporation of Delaware No Drawing. Application October 10, 1952Serial No. 314,204

7 Claims. (Cl. 260--810) The present invention relates to a method ofretarding or preventing the deterioration of a rubber due to aging orexposure to the atmosphere and to the rubber compositions so obtained.It has long been known that such deterioration can be greatly retardedby treating the rubber before or after vulcanization with certainsubstances known as age resistors or antioxidants. The present inventionprovides a new class of antioxidants for rubber which have the advantageof not discoloring the rubber. They are, accordingly, useful for theproduction of light colored rubber goods.

The antioxidants or age resistors of this invention are trisphenols. Inthese compounds one of the hydrogen atoms in each of the methyl groupsof 2,6-xylenol is replaced by a phenolic substituent. This class ofcompounds may be called D ni-bis(hydroxyphenyl)-2,6-xylenols. Thephenolic nuclei may be further substituted. In fact it is preferred thateach nucleus contain at least one methyl group. Compounds of this classsubstituted by alkyl groups containing one to four carbon atoms oraralkyl groups or chlorine have also exhibited antioxidant activity butincreasing the size of the substituent above methyl or replacing themethyl group by chlorine has in general lowered the activity. Thetrisphenols which have exhibited optimum properties may be representedby the structural formula stocks suitable for manufacture of whiterubber goods were compounded comprising Stock A B 0 Parts by weight Palecrepe rubber 100 100 100 Zinc oxide 60 60 60 Lithopone. 2O 2O Sulfur 2 22 Diphenylguanidine phthalate. 0.820 0.820 0.820 2-Benzothiazolylthiolbenzoate 0. 625 0.625 0. 625 Paraffin 0. 250 0. 250 0.250 fia-Bis(ti-hydroxy-m-tolyDm tol 1. 0 M r-Bis(d-hydroxy-3,5-xylyl)mesitol 1. 0

The stocks were cured in the usual manner by heating in a press at 126C. and then artificially aged by heat- 2,841,627 Patented July 1, 1958ICC ing in a bomb at 121 C. under pounds air pressure per square inch.The physical properties of the cured compositions before and after agingare set forth below:

Table I Modulus of Cure Elasticity in Tensile at Ult. Stock Hrs. AgedTime in lbs/in. at Break in Elong.

Mins. Elongation lbs./i11. percent The same stocks were alsoartificially aged by heating in a bomb at 70 C. under 300 pounds oxygenpressure per square inch. As illustrative of the antioxidant activity,the percent of the original tensile strength of the 60 minute cureretained after aging is set forth in the first column of the tablebelow. Additionally, the resistance to discoloration of the compositionscontaining the antioxidants as compared to the untreated stock wasevaluated by means of a photovolt reflectance meter calibrated against astandard of magnesium oxide as 100. The percent of light reflected fromthe surface of the cured stocks after 10 days exposure to an 8-1 sunlampis set forth in the second column. It will be noted that theincorporation of the antioxidants resulted in no decrease at all in thelight reflected.

Table 11 Percent of Original Percent Stock Tensile Light RetainedReflectance after Aging As has been stated, replacement of one or moreof the methyl groups by chlorine is permissible. For example antioxidantactivity was exhibited by 4-chloroa ed-bis(5-chloro-2-hydroxyphenyl)2,6-xylenol, (1 ,04 bis(5-chloro-Z-hydroxyphenyl)mesitol and a ,a-biS(5- chlcro-2-hydroxy-m-tolyl)mesitol. However, the antioxidantactivity was considerably weaker than that exhibited by the foregoingcompounds.

As further specific embodiments of the invention rubber stocks werecompounded employing the same base composition as described above andcompared to the untreated base. Thus, the composition of Stock D belowis the same as Stock A above but Stock D was cured and aged at the sametime as Stocks E and F.

These stocks were cured in a press at 126 C. and artificially aged byheating in a bomb at 121 C. under 80 pounds air pressure per squareinch. The modulus and tensile properties before and after aging ascompared to the base containing no antioxidant are set forth below:

Table III Modulus of Cure Elasticity Tensile at Ult. Stock Hrs. Time inii1.lbs./in. at Break in Elong, Aged Mins. Elongation lbs/in. Percent Inaddition the resistance of the stocks to discoloration was determined inthe manner described after 10 days exposure to an 8-1 sunlamp.

Table IV Cure Time Pei-eent Stock in Mins. Light Reflectance As a stillfurther specific embodiment of the invention a rubber stock wascompounded by adding one part of antioxidant to the base above describedand the properties before and after aging compared to the base stockdesignated as G in the series:

Stock Antioxidant G II Parts by Weight None a ,a"-Bis(6 hydroxy-m-tolyl)4 tert butyl 2,6-

xylenol 1. O

The stocks were cured in a press at 126 C. and the vulcanizatesartificially aged by heating at 100 C. for 96 hours in a circulating airoven. The modulus and tensile properties before and after aging were asfollows:

Similarly, antioxidant activity in a rubber composition was exhibited bya ad -bis(5-tert-butyl-2-hydroxy-mto1yl)mesitol although the activitywas reduced somewhat by the presence of the larger groups. Furthermore,having the hydroxyl group present in the 4- instead of the 2-positiondid not destroy the antioxidant activity although a ,oc -biS(5-trt butyl4 hydroxyl m tolyl)- mesitol was significantly weaker.

Although some of the antioxidants are new chemical compounds, they maybe prepared by methods analogous to those described in the chemicalliterature. Typical examples of the preparation and properties of someof the compounds are described below.

The sodium salt of 2-hydroxy-a ,a -mesitylenediol was prepared bycondensing formaldehyde and p-cresol in the molecular ratio of 2:1 in25% aqueous caustic soda. After two days at room temperature or twohours at 100 C. the reaction was complete and the lustrous plates weredissolved in water and the solution acidified to a pH of about 6 with1:2 acetic acid. 2-hydroxya ,o -mesitylenediol was obtained in 93% yieldas brilliant white prisms from ethyl acetate, M. P, 128.4129.1 C. 16.8parts by weight of the 2-hydroxy-a ,a -mesity1- enediol so prepared wasadded to a two molar excess of p-cresol and 2 parts by weighthydrochloric acid added thereto. The temperature rose steadily to about95 C. due to the heat of reaction. The reaction was essentially completein about ten minutes. The mixture was then cooled, slurried withpetroleum ether and filtered. The a ,a -bis(6-hydroxy-m-tolyl)mesitolwas obtained in a 92% yield as white prisms, from ethyl acetate, M. P.215.2-2l6.0 C.

From the reaction of 2-hydroxy-ot ,a -mesitylenediol and2,4-di-tert-amylphenol there was obtained m ,a -bis(2-hydroxy-3,S-di-tert-amylphenyl)mesitol, M. P. 85.6-86.0 C. although inpoor yieid.

Reaction of Z-hydroxy-a ,a -mesitylenediol and 2,4- xylenol gave thedesired a ,ot -bis(6-hydroxy-3,5-xylyl)- mesitol as a white crystallinesolid, M. P. l83.6l84.2 C.

Condensation of Z-hydroxy-a ,r t mesitylenediol with4-tert-butyl-o-cresol gave a 71.5% yield of a ,a bis(5-tert-butyl-Z-hydroxy-m-tolyl)rnesitol obtained as white rosettes frombenzene, M. P. 192193 C.

Condensation of 2-hydroxy-a ,:fi-mesitylenediol withp-ot,OL-dlmEthYIbBHZYlPhCIIOI gave a ,ot-bis(5-ot,ot-dimethylbenzyl-Z-hydroxypherryl)mesitol. This product was afine white powder, M. P. 2182l9 C.

Condensation of Z-hydroxy-a ,rr -mesitylenediol witho-tert-butyl-o-cresol in toluene solution employing hydrochloric acid ascondensation catalyst gave a ,ot -biS(5tert-butyl-4-hydroxy-m-tolyl)mesitol. The product after recrystallizingfrom petroleum ether was a white powder, M. P. 133-134 C.

5-tert-butyl-2-hydroxy-mxylene-or,a'-diol was prepared by treatingp-tert butylphenol with an excess of aqueous formaldehyde and strongalcoholic caustic. The mixture was allowed to stand for three days atroom temperature and the fine white needles separated by filtration andrecrystallized from petroleum ether, M. P. 88.2-89.0 C. The addition ofhydrochloric acid to a mixture of pcresol and the2,6-dimethylol-p-tert-butylphenol so prepared at 45 C. gave a good yieldof e ,ot -bis(6-hydroxym-tolyl)-4-tert-butyl-2,6-xylenol as nacreousplates from petroleum ether, M. P. l35136 C.

fie -bis(6-hydroxy-m-tolyl)durenol Was obtained in good yield bycondensing p-cresol with 6-hydroxy-rx ,ot durenediol. The product was afine white crystalline powder from benzene, M. P. 191-192 C.

Although natural rubber has been selected to illustrate the invention,the term a rubber is employed in a generic sense to define a sulfurvulcanizable polymer and includes reclaimed rubber, balata, gutta perchaand synthetically prepared rubber diene polymers, as for example abutadiene-l,3 polymer, copolymers of butadiene- 1,3 with styrene oracrylonitriie and the like. In general the antioxidants herein describedare effective for the preservation of organic substances whichdeteriorate by absorption of oxygen from the air.

H H H 0 0 0 11001100 Hr-OR CH1 Hr CHI where R is selected from the groupconsisting of hydrogen and methyl groups.

3. A vulcanized diene polymer rubber preserved by having incorporatedtherein a compound of the structure H H H 0 o 0 00110-0Hr0 CH: CH: CH:

4. A vulcanized diene polymer rubber preserved by having incorporatedtherein a compound of the structure H H H o o 0 omO-wm-O-cH'OoH, C H; 0HI 0 H] 5. Unvulcanized rubber having incorporated therein a compound ofthe structure H H n o o o ROCHUCIDOR on, em on,

where R is selected from the group consisting of hydrogen and methylgroups.

6. Unvulcanized rubber having incorporated therein a compound of thestructure 11 H H o 0 0 cmOonOom-Oom CH1 CH CH! 7. A compositioncontaining a sulfur vulcanizable diolefinic rubbery polymer and as anantioxidant a compound of the formula:

0H 0H 0H OHQCHIQCHPQCHI HI Hi HI References Cited in the file of thispatent UNITED STATES PATENTS Newly Feb. 17, 1953 OTHER REFERENCES TheIndia Rubber Journal," October 24, 1931, pages 535 and 536.

Ind. Eng. Chem. 41, 1442-1447 (July 1949).

2. A VULCANIZED DIENE POLYMER RUBBER PRESERVED BY HAVING INCORPORATEDTHEREIN A COMPOUND OF THE STRUCTURE